Re: [RFC PATCH 2/3] btrfs-progs: kernel based default features for mkfs

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Anand Jain wrote on 2015/10/21 16:45 +0800:
mkfs from latest btrfs-progs will enable latest default features,
and if the kernel is down-rev and does not support a latest default
feature then mount fails, as expected.

This patch disables default features based on the running kernel.

Signed-off-by: Anand Jain <anand.jain@xxxxxxxxxx>
---
  mkfs.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mkfs.c b/mkfs.c
index a5802f7..2b9d734 100644
--- a/mkfs.c
+++ b/mkfs.c
@@ -1357,10 +1357,13 @@ int main(int ac, char **av)
  	int dev_cnt = 0;
  	int saved_optind;
  	char fs_uuid[BTRFS_UUID_UNPARSED_SIZE] = { 0 };
-	u64 features = BTRFS_MKFS_DEFAULT_FEATURES;
+	u64 features;
  	struct mkfs_allocation allocation = { 0 };
  	struct btrfs_mkfs_config mkfs_cfg;

+	features = btrfs_features_allowed_by_kernel();
+	features &= BTRFS_MKFS_DEFAULT_FEATURES;
+

Despite the problem of btrfs_features_allowed_by_kernel() I mentioned in previous mail,
the behavior is a little aggressive for me.

So a user with old kernel won't be able to create a filesystem with newer feature forever.
Maybe the user are just making btrfs for his or her newer kernel?

IMHO, it's better to output a warning other than just change features without any information.

Thanks,
Qu

  	while(1) {
  		int c;
  		static const struct option long_options[] = {

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